This invention relates to an improved hydraulic driving circuit for a forklift truck in which the rate of descent of the inner masts may be controlled and sudden descent of the inner masts may be prevented from occurring upon breakage of piping sections connecting the left and right side lift cylinders, the forklift truck being of the type in which the inner masts may be lifted or lowered by the operation of the left and right lift cylinders.
The prior-art hydraulic circuit associated with the forklift truck is shown in FIG. 1. Between hydraulic piping sections 10a' and 10b' providing hydraulic communication between the bottom chambers of the left and right side hydraulic lift cylinders 4' and 5', there is provided a flow control valve 13' for controlling the rate of descent of the inner masts (not shown). In a bottom chamber of the left side cylinder 4' there is mounted a safety descent valve 20' for preventing the sudden descent of the inner masts (not shown). The valve 20' is mounted from the bottom end surface of the cylinder 4.
In the above described conventional hydraulic circuit, difficulties are encountered at the time of removing and mounting the flow control valve 13' and/or the safety descent valve 20' for inspection or repair. That is, when replacing the flow control valve 13', the piping sections 10a' and 10b' connecting the valve 13' and the respective lift cylinders 4' and 5' to each other and the piping section 12' connecting the flow control valve 13' and an ascent/descent changeover valve 11' to each other need to be dismounted in advance by a manual operation including a number of separate operating steps. When replacing the safety descent valve 20', a suitable tool needs to be introduced and manipulated through an operating through-hole 2a' in a lower tie beam 2' of the outer masts 1' by a laborious and time-consuming operation. In addition, in a certain conventional mast device for a forklift truck, a mast support 29' is provided on the lower surface of the lower tie beam 2' (as shown in FIG. 5). With the mast device, the lift cylinder 4' needs to be dismounted from the lower tie beam 2' before proceeding to the dismounting of the safety descent valve 20'.